Transition metal complexes or coordination complexes are
molecules that contain groups arranged around a central metal ion. In a
way, these are like "lego-molecules", easily assembled from smaller parts, and
sometimes they are easily transformed into new molecules by switching out old
parts for new ones. That rapid assembly and disassembly is part of what
makes these comppounds very useful in both industrial and biological catalysis.

What are the building blocks that go into a
coordination complex? The key, central part is usually a transition metal
ion (although cations from other parts of the periodic table are also seen in
some cases). The following table shows some common examples of ions formed
by each of the transition metals.

Table TM2.1. Some common transition
metal ions.

Sc3+

Ti4+Ti3+

V5+V4+

Cr3+
Cr6+Cr2+

Mn2+Mn4+Mn7+

Fe3+Fe2+

Co2+Co3+

Ni2+

Cu2+Cu+

Zn2+

Y3+

Zr4+

Nb5+Nb3+

Mo6+
Mo4+Mo5+

(Tc7+)*
(Tc4+)*

Ru3+
Ru4+Ru2+

Rh3+Rh+

Pd2+Pd4+

Ag+Ag2+

Cd2+

La3+

Hf4+

Ta5+

W6+
W4+W5+

Re4+
Re6+Re7+

Os4+Os3+

Ir4+Ir3+

Pt2+Pt4+

Au3+Au+

Hg2+Hg1+

*radioactive, with a short half-life; seldom observed.

For each transition metal, the most common form of ion is listed first.
For example, iron is often found in compounds as Fe3+. Other
common ions are also shown below that; iron is seen pretty frequently as Fe2+.
Other charges are possible; iron has been reported with charges all the way from
Fe0 to Fe6+, but these cases are less common.

Also, it's worth noting that the top row of transition metals is generally much
more common than the next two rows. That doesn't mean that they are less
important; gold (Au), silver (Ag) and platinum (Pt) are certainly important
economically. However, you may be more likely to encounter examples of
complexes from the first row. That's especially true in biological
chemistry, because organisms have evolved to make use of those metal ions that
are most readily available to them.

Problem TM2.1.

a) The metals on the left hand side of the table tend to have relatively high
charges compared to the metals on the right. What do ions such as Sc3+,
Zr4+, and Ta5+ have in common that would explain this
trend?

b) The metals on the right side of the table have relatively low charges.
In nature, metals such as copper (Cu), silver (Ag), and gold (Au) are frequently
found as native metals (e.g. Cu0, with no charge at all) rather than
as compounds. Explain this preference for low charges in this part of the
transition metals.

c) The metals in the middle, on the other hand, have very wide ranges of
charges (that's why the table lists three common charges for those, although
even more exist). Why?

The charge on the metal ion is sometimes called the oxidation state.
This term refers to the fact that metals that are exposed to the elements
sometimes become positively charged, forming compounds such as metal oxides.
The oxygen from the air provides the oxide or hydroxide ion to counter the
charge when the metal atom loses electrons and becomes a metal cation.
Familar examples include the oxidation of aluminum metal to form silvery-white
aluminum oxide; you may have seen aluminum screen doors that are actually
covered in a coating of aluminum oxide. The Statue of Liberty was
originally covered in copper metal, but quickly became coated in money-green
copper oxide.

Sometimes, within a complex, the charge or oxidation state of the metal is
denoted using Roman numerals. For example, Co3+ is sometimes
writted Co(III); Mn2+ might be written Mn(II).

After that, we won't need to worry about it. Sometimes osmium is found as
Os(VIII), for example, but we don't see a 9+ charge very often.

The metal ion is the first building block. The second building block is
the ligand. The ligands are the pieces that are arranged around the metal
ion. On the last page, we saw chloride anions (Cl-) and ammonia
(NH3) act as ligands in different transition metal complexes.
It is easy to imagine how a positively charged metal ion would attract a
negatively charge anion such as chloride. As it happens, neutral ligands
are just as common. The main requirement for sticking to a metal ion is a
non-bonding pair of electrons, or a lone pair (at least, that's the case at this
stage of your education).

The following table illustrates a variety of ligands for transition metal
complexes.

Table TM2.2. Some common ligands.

Problem TM2.2.

Calculate the overall charge on the following complexes, if any.

a) [Fe(II)(OH2)6]
b) [Cr(III)(NH3)5Cl] c) [(py)4Mn(II)(SCN)2]

d) [Fe(II)(CN)6] e)
[Co(III)(CN)5CO] f) [Fe(II)(CN)5NH3]

A coordination complex has a central metal ion with a number of ligands arranged
around it. The last piece is the counterions, which would balance out the
charge of the transition metal complex ion. We saw examples of common
counterions on the introduction page.

Problem TM2.3.

Calculate the charge or oxidation state on the metal ion in each of the
following complexes.

a) [Cr(OH2)6](NO3)3
b) K3[FeF6] c)
[Cr(SCN)(NH3)5]SO4

d) K4[Mn(CN)6]
e) [Au(NCCH3)2]ClO4 f)
[(Ph3P)Ag(CN)]

On the next page, we will see how some ligands can bind to a metal more than
once. That helps them hold on more tightly.

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's
University (with contributions from other authors as noted). It is freely
available for educational use.